ceph: fix a memory leak on cap_auths in MDS client

The cap_auths that are allocated during an MDS session opening are never
released, causing a memory leak detected by kmemleak.  Fix this by freeing
the memory allocated when shutting down the MDS client.

Fixes: 1d17de95 ("ceph: save cap_auths in MDS client when session is opened")
Signed-off-by: Luis Henriques (SUSE) <luis.henriques@linux.dev>
Reviewed-by: Xiubo Li <xiubli@redhat.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>